1. Field of the Invention
The present invention relates to wireless communication, and more particularly, to a method and apparatus for transmitting a frame in a Wireless Local Area Network (WLAN) system.
2. Discussion of the Related Art
With the recent development of information communication technology, a variety of wireless communication techniques are being developed. From among the techniques, WLAN is a technique, enabling wireless access to the Internet at homes or companies or in specific service providing areas through mobile terminals, such as a Personal Digital Assistant (PDA), a laptop computer, and a Portable Multimedia Player (PMP), based on radio frequency technology.
Lots of standardization tasks are being carried out since Institute of Electrical and Electronics Engineers (IEEE) 802 (i.e., the standard organization for WLAN technique) was set up on February, 1980. The initial WLAN technique was able to support the bit rate of 1 to 2 Mbps through frequency hopping, band spreading, and infrared communication by using a 2.4 GHz frequency band in accordance with the IEEE 802.11 standard, but the recent WLAN technique can support a maximum bit rate of 54 Mbps using Orthogonal Frequency Division Multiplex (OFDM) method. Furthermore, in the IEEE 802.11 standard, the standardization of various techniques, such as the improvements of Quality of Service (QoS), the compatibility of Access Point (AP) protocols, security enhancement, radio resource measurement, wireless access vehicular environment for vehicle environments, fast roaming, a mesh network, interworking with an external network, and wireless network management, is put to practical use or being developed. Furthermore, in order to overcome a limit to the communication speed that has been considered as vulnerabilities in the WLAN technique, IEEE 802.11n has recently been standardized as a technology standard. The object of the IEEE 802.11n is to increase the speed and reliability of a network and to expand the coverage of a wireless network.
More particularly, the IEEE 802.11n standard is based on a Multiple Inputs and Multiple Outputs (MIMO) technique which uses multiple antennas on both sides of a transmitter and a receiver, in order to support a High Throughput (HT) having a data processing speed of 540 Mbps or higher, minimize transmission error, and optimize the data rate. Furthermore, the IEEE 802.11n standard may use not only a coding scheme for transmitting several redundant copies in order to increase reliability of data, but also an Orthogonal Frequency Division Multiplex (OFDM) scheme in order to increase the data rate.
With the spread of the WLAN technique being activated and applications using the WLAN technique being diversified, there is a need for a new WLAN system capable of supporting the throughput higher than the data processing speed supported by the IEEE 802.11n standard. However, an IEEE 802.11n Medium Access Control (MAC)/Physical Layer (PHY) protocol is not effective in providing the throughput of 1 Gbps or higher. This is because the IEEE 802.11n MAC/PHY protocol is for the operation of a station (STA) having a single Network Interface Card (NIC). Accordingly, if the throughput of frames is increased while the existing IEEE 802.11n MAC/PHY protocol remains intact, overhead is increased. Consequently, to improve the throughput of a wireless communication network while the existing IEEE 802.11n MAC/PHY protocol (i.e., the single STA architecture) remains intact is limited.
In order to achieve the data processing speed of 1 Gbps or higher in a wireless communication network, there is a need for a new system which is different from the existing IEEE 802.11n MAC/PHY protocol (i.e., the single STA architecture). A Very High Throughput (VHT) WLAN system is the next version of the IEEE 802.11n WLAN system. The VHT WLAN system is one of the recent IEEE 802.11 WLAN systems which are being newly proposed in order to support the data processing speed of 1 Gbps or higher in a MAC Service Access Point (SAP).
The VHT WLAN system enables a plurality of VHT STAs to access radio channels at the same time in order to efficiently use the channels. To this end, the VHT WLAN system supports transmission of a Multi-User Multiple Inputs Multiple Outputs (MU-MIMO) method using multiple antennas. A VHT Access Point (AP) can perform a Spatial Division Multiple Access (SDMA) transmission method of transmitting spatially multiplexed data to a plurality of VHT STAs. If a plurality of spatial streams is distributed into a plurality of STAs and transmitted at the same time using a plurality of antennas, the entire throughput of a WLAN system can be increased.
Legacy terminals, supporting WLAN systems (e.g., IEEE 802.11a/b/g) anterior to the IEEE 802.11n WLAN system, and HT terminals supporting the IEEE 802.11n WLAN system may be basically operated in an active mode and a Power Saving (PS) mode. A terminal which is stably supplied with power using a power cable is relatively less sensitive to consumption efficiency because the power is stably supplied. On the other hand, a terminal operated by the battery of a certain capacity may be sensitive to power consumption efficiency because it must be operated within the limited power. From a viewpoint of terminal mobility, a terminal which is supplied with stable power through a power cable may have a limit to mobility. On the other hand, a terminal supplied with power from the battery may be less sensitive to mobility. In order to increase the power consumption efficiency of a terminal, a terminal may be operated in the PS mode. A terminal operating in the PS mode repeatedly switches between an awake mode and a sleep mode in order to efficiently use limited power.
Consideration to power consumption efficiency may still be an important issue even in the VHT WLAN system. Accordingly, a new Physical Layer Convergence Procedure (PLCP) frame format and a method of determining and transmitting control information to be transmitted through a PLCP frame need to be taken into consideration by taking power consumption efficiency into consideration in a WLAN system.